1. Field of the Invention
The present invention relates to an electric power steering (EPS) apparatus for vehicles that uses an electric motor to assist the manual steering force applied by a driver. More particularly, this invention relates to an electric power steering apparatus with a function reducing vibration and noise.
2. Description of Related Art
Lately, almost all vehicles are equipped with a power steering apparatus that assists the steering force applied by a driver.
As one example of such a power steering apparatus, an electric power steering (EPS) apparatus is known. The known electric power steering apparatus includes, for example, a torque sensor for detecting steering torque applied to wheels, and an electric motor for steering assistance, and is structured to drive the electric motor in response to the steering torque detected by the torque sensor. Rotational output of the electric motor serves as steering-assist torque, and is decelerated by a gear mechanism and then transmitted to the output shaft of the steering mechanism. Accordingly, the transmitted rotational output assists the steering force applied to the steering wheel, thereby steering the vehicle.
Conventionally, brush motors were mainly used as the electric motors for the above-described conventional electric power steering apparatus. However, with the enhancement of reliability and further improvement in efficiency, the use of brushless motors has increased, and the demand for high-powered brushless motors is growing.
Since the electric motor is positioned close to the steering wheel in cases where a column-type electric power steering apparatus has an electric drive motor attached to the steering column section, it is necessary to suppress and reduce noise and vibration caused by the electric motor.
Meanwhile, there is also the demand for suppression of torque loss fluctuation in the above-described electric motor. Torque loss means loss of an electric motor's output torque caused by friction arising from the motor structure, and other electromagnetic factors. Torque loss adds unintended force or opposite-direction force to steering direction even in the case of slight steering during straight-ahead driving, and degrades the steering sensation.
Particularly when torque loss fluctuation is large, the fluctuation turns into physical vibration, which is then transmitted to the steering wheel, causing cogging. Such cogging brings a bad or uncomfortable feeling to a driver, and thus it is necessary that an electric motor used for an electric power steering apparatus generates small torque loss fluctuation.
As described in Japanese Patent Laid-Open (Kokai) Publications No. SHO62-110468 and No. 2001-275325, there is a known method for reducing torque loss fluctuation by adjusting the combination of the number of stator salient poles with the number of rotor magnetic poles. Furthermore, as described in Japanese Patent Laid-Open (Kokai) Publication No. 2003-250254, an electric power steering apparatus having an electric motor where the torque loss fluctuation is reduced by the above-described method is also known.